1. Field of the Invention
The present invention relates to a picture transmission system. More specifically, it relates to a picture transmission system adapted to a visual telephone and the like, which transmits variable-density pictures through a public telephone line and the like by a variable sampling rate system.
2. Description of the Prior Art
In a visual telephone, picture information is generally subjected to band compression coding for transmitting variable-density still pictures. Such band compression coding is performed by methods utilizing intraframe correlation and interframe correlation. The methods utilizing the intraframe correlation include differential pulse-code modulation (DPCM) of predicting subsequent sampling values from some sampling values for coding predictive errors thereof to transmit the same and a variable sampling rate coding system.
In such a visual telephone, predictive errors which are nonlinearly quantized by DPCM are inevitably required to be in excess of 4 bits for obtaining practical restored pictures, leading to reduction in the compression effect.
On the other hand, the variable sampling rate coding system is adapted to compress preposition predictive errors further in the time base direction to transmit the same, and is remarkably effective for improving compression efficiency. Description is now made of the variable sampling rate coding system.
FIGS. 1 and 2 are schematic block diagrams showing principle structure of a conventional variable sampling rate coding system, and FIG. 3 is an illustration showing an example of variable sampling rate coding.
Referring now to FIG. 1, input signals are subjected to subtraction by predictive decoding signals supplied from a local predictive decoding circuit 1 and a subtractor 2, whereby error signals are supplied to a variable sampling rate coding circuit 3. This variable sampling rate coding circuit 3 has such a quantization characteristic as shown in FIG. 3. Namely, the variable sampling rate coding circuit 3 defines relations between sampling periods and sampling values in the form of triangles, which are scrolled in the horizontal direction on the screen as shown in FIG. 3. In this case, time difference values are increased as amplitude difference values are reduced, followed by increase in compressibility. To the contrary, compressibility is lowered in positions in which the amplitude difference values are large and time difference values are small. Thus, the relations between the sampling periods and the sampling values are defined by the triangles, whereby differences as indicated by arrows are transmitted as coding output series through a buffer memory 4. The coding amplitude components are supplied to the local predictive decoding circuit 1, to be subjected to feedback to the subtractor 2 as amplitude predictive decoding signals.
On the other hand, the received coding amplitude components are supplied to an amplitude predictive decoding circuit 6 and a sampling point decoding circuit 7 through a buffer memory 5 as shown in FIG. 2. The sampling point decoding circuit 7 has the same characteristics as the variable sampling rate coding circuit 3, and creates sampling space components from the transmitted coding amplitude components, for integrating the same to regenerate sampling point signals. The sampling point signals and amplitude signals decoded from the amplitude predictive decoding circuit 6 are supplied to a sampling point regenerative circuit 8, which in turn regenerates sampling points determined by the both signals. The regenerated sampling points are supplied to an interpolation circuit 9, which in turn performs appropriate interpolation (e.g., linear interpolation) thereby to regenerate signals.
The aforementioned variable sampling rate system is remarkably effective for improving compression efficiency, whereas lowered is the quality of pictures by errors. In other words, in the variable sampling rate system, the quantized sampling series itself includes information on sampling points (time difference values), and the time difference values are increased as the sampling values are reduced, as is obvious from FIG. 3. The compressibility is increased when the sampling values are thus small, and hence errors horizontally caused with increased compressibility are inevitably propagated in the horizontal direction. Consequently, outlines of the pictures regenerated in the receiving area are made irregular along respective scanning lines, leading to deterioration in the picture quality.